Various methods and apparatus for the compression molding of glass optical elements are known in the prior art. With these methods and apparatus, optical element preforms sometimes referred to as gobs are compression molded at high temperatures to form glass lens elements. The basic process and apparatus for molding glass elements is taught in a series of patents assigned to Eastman Kodak Company. Such patents are U.S. Pat. No. 3,833,347 to Engle et al, U.S. Pat. No. 4,139,677 to Blair et al, and U.S. Pat. No. 4,168,961 to Blair. These patents disclose a variety of suitable materials for construction of mold inserts used to form the optical surfaces in the molded optical glass elements. Those suitable materials for the construction of the mold inserts included glasslike or vitreous carbon, silicon carbide, silicon nitride, and a mixture of silicon carbide and carbon. In the practice of the process described in such patents, a glass preform or gob is inserted into a mold cavity with the mold being formed out of one of the above mentioned materials. The molds reside within a chamber in which is maintained a non-oxidizing atmosphere during the molding process. The preform is then heat softened by increasing the temperature of the mold to thereby bring the viscosity of the preform into the range from about 10.sup.10 P to about 10.sup.6 P. Pressure is then applied to force the preform to conform to the shape of the mold cavity. The mold and preform are then allowed to cool below the glass transition temperature of the glass. The pressure on the mold is then relieved and the temperature is lowered further so that the finished molded lens can be removed from the mold.
With regard to the compression molding of near-net-shape glass optical elements it is well known that a glass preform with a precision polished surface must be pressed between the upper and lower halves of a mold. If a double positive lens (convex-convex lens) is to be molded, for example, a spherical or oblate spheroid glass preform of the proper volume is placed between the mold halves, heated until the glass has a viscosity in the range of 10.sup.6 -10.sup.10 Poise, and is compressed until the mold is closed then preferably cooled to a temperature below the annealing point and demolded. In such an arrangement, as shown in FIG. 1, the upper and lower mold halves 10, 12 compress a spherical glass preform 14 therebetween. The radius of the spherical glass preform 14 must be less than the radius of both of the concave mold surfaces 16, 18. As the glass preform 14 is compressed, the glass flows generally radially outwardly from the center of the mold cavity thereby expelling any gas from the mold cavity. This results in the production of a double convex lens 20 free from distortion due to trapped gas. Such molded lenses typically have accurate and repeatable surface replication relative to the mold.
Depending on the final shape of the lens to be formed, specially shaped preforms are sometimes required to ensure that the glass flows from the center of the mold cavity to the edge as shown in FIGS. 2-4. FIG. 2 schematically depicts a prior art arrangement wherein the upper mold half 22 includes a plano mold surface 24 and the lower mold half 26 includes a concave mold surface 28. In such an arrangement, a spherical preform 30 just as with the arrangement depicted in FIG. 1, but in this instance to produce a piano-convex optical element 32. However, looking at FIG. 3 there is schematically depicted a prior art arrangement wherein the upper mold half 34 includes a convex mold surface 36 and the lower mold half 38 includes a concave mold surface 40. In such an arrangement, it is preferred to use a plano-convex preform 42 to produce a concave-convex optical element 44. The radius of the convex surface of preform 42 must be less than the radius of concave mold surface 40. This ensures first contact between mold surface 40 and preform 42 substantially at the centerline of the mold apparatus thereby causing the preform to flow generally radially outwardly to prevent the trapping of gases. Similarly, the first contact between convex mold surface 36 and the piano surface of preform 42 is substantially at the centerline of the mold apparatus thereby also causing the preform 42 to flow generally radially outwardly to prevent the trapping of gases. FIG. 4 schematically depicts a prior art arrangement wherein the upper mold half 46 includes a convex mold surface 48 and the lower mold half 50 includes a convex mold surface 52. In such an arrangement, it is preferred to use a plano-plano preform 54 to produce a double concave optical element 56. The plano-plano preform 54 ensures first contact between the mold surfaces 48, 52 and preform 54 substantially at the centerline of the mold apparatus thereby causing the preform to flow generally radially outwardly to prevent the trapping of gases. Examples of such practices are cited in U.S. Pat. Nos. 5,662,951 and 4,797,144. The method outlined in these patents works well for single cavity molds where one lens is molded from one preform. When molding an array of lenses or microlenses from one preform, the above approach will trap gas causing surface distortion of the lenses. U.S. Pat. No. 5,276,538 indicates that an array of microlenses may be fabricated by pressing a plano preform between an upper plano mold surface and a lower mold surface with concave microlens cavities. This approach, however, will cause surface figure distortion of the microlens features due to trapped gas. Another method of forming an array of microlens is taught in U.S. Pat. No. 5,276,538 where micro-sized spherical preforms are placed in a plurality of cavities of the lower mold and many microlenses arc molded simultaneously. However, due to the expensive fabrication costs of the spherical performs and the production time required to place many microspheres onto a mold, this method would be cost prohibitive.